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audible signal to switch?

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Anthony Mariana

New Member
How would you convert an anolog signal into a on/off switch?a little background. i would like to use a metal detector (hobby type) on a conveyor system. when metal is detected i would like it to signal and shut down the conveyor. aside from the speaker there is also a headphone plug.

I was told that this was easy
The simplest way would be to use a LM339 comparator IC.:

To reduce the interference element, the speaker headphone would be preferable to use.

A small diode bridge would be used to bring the audio signal all in positive territory. A small capacitor could be used to smooth the "DC" signal (not too big a capacitor or the response time of the system might become to slow. With LM339 and a little variable DC level (a knob on a potentiometer), you could be able to set the sensitivity.

You will need something to latch that metal detection. That can be made with logic gates: Or with relay logic.
If you don't implement a latch, the conveyor may start right after it detected something, barely slowing down.

Problem....Can anyone show me how they connect ? Thanks
 

MikeMl

Well-Known Member
Most Helpful Member
Audio detector and latching comparator. Close switch momentarily to reset it once tripped. As shown, it detects about 6 cycles of a 2Vp-p 1kHz sine input.

If you can tell me what the frequency/amplitude of your tone burst is, I can optimize it for that.
If you need to drive a relay or some other load, a buffer may be needed. The "LatchedAlarm" output signal is not suitable for driving a relay directly.
 

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ethan169

New Member
Audio detector and latching comparator. Close switch momentarily to reset it once tripped. As shown, it detects about 6 cycles of a 2Vp-p 1kHz sine input.

If you can tell me what the frequency/amplitude of your tone burst is, I can optimize it for that.
If you need to drive a relay or some other load, a buffer may be needed. The "LatchedAlarm" output signal is not suitable for driving a relay directly.
This is an interesting idea!

On another note i dont mean to steal the topic away and go off on a tangent... but Mike what program did you do this schematic and simulation in?

Thanks

-Ethan
 

MikeMl

Well-Known Member
Most Helpful Member
U1 is a generic comparator/opamp, but it is a good representation of a LM339.

The program is the free LTSpice (search these forums or google it)

You need to determine the minimum amplitude (Vp-p) that you want the circuit to respond to. If is less than 2Vp-p, we may have to add an opamp amplifier ahead of the rectifier.
 

Hero999

Banned
You might also want to consider Schottky diodes which will also improve the sensitivity.
 
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